A typical wireless communication system includes one or more base stations, each providing one or more coverage areas or cells in which to serve user equipment devices (UEs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped devices, whether or not user operated. Further, each base station of the system may then be coupled or communicatively linked with network infrastructure such as a switch and/or a gateway that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a UE within coverage of the system may thus engage in air interface communication with a base station and thereby communicate via the base station with various remote network entities or with other UEs served by the system.
Such a system may operate in accordance with a particular air interface protocol, with communications from a base station to UEs defining a downlink or forward link and communications from the UEs to the base station defining an uplink or reverse link. Examples of existing air interface protocols include, without limitation, Long Term Evolution (LTE) (using Orthogonal Frequency Division Multiple Access (OFDMA) on the downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) on the uplink), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), Global System for Mobile Communication (GSM), WI-FI, and BLUETOOTH. Each protocol may define its own procedures for registration of UEs, initiation of communications, handover between cells, and other functions related to air interface communication.
Under the air interface protocol, each base station coverage area may operate on a carrier, which could be either frequency division duplex (FDD) or time division duplex (TDD). An FDD carrier defines separate frequency channels for downlink and uplink communication, so that UEs can engage in downlink and uplink communication concurrently. Whereas, a TDD carrier defines a single frequency channel multiplexed over time between downlink and uplink use, so that UEs can engage in downlink communication in only particular time intervals and uplink communication in only other time intervals.
Further, each carrier in such a system could define a number of sub-channels for carrying information between the base station and UEs. For instance, each carrier could define a reference channel on which the base station broadcasts a reference signal that UEs can evaluate to determine coverage quality. And each carrier may define various other downlink and uplink control channels and traffic channels for carrying control signal and bearer traffic (e.g., application-layer traffic) between the base station and UEs.
When a UE initially enters into coverage of such a system, the UE may scan for a strongest coverage area. The UE may then read system information broadcast from the base station that provides that coverage area and may engage in a process to register for service by the base station and generally by the system. For instance, the UE and base station may engage in signaling to establish a radio-link-layer connection, the UE may transmit an attach request, and the base station and/or supporting infrastructure may authenticate and authorize the UE for service and establish a record indicating where in the system the UE is operating. The UE may then be served by the base station.